This talk will present a series of studies that have investigated the impact of amount of bilingual exposure on the development of each language of bilingual children in the areas of vocabulary, grammar and language processing. Data will be presented on preschool and schoolage learners of English and French in Canada as well as school-age second language learners of Icelandic from various L1 backgrounds. The studies look at the effect of amount of exposure and the effect of its timing (the age at which bilingual exposure started) by comparing simultaneous and sequential learners while controlling for the overall amount of exposure. The findings also show that the relative effects of amount and timing are not uniform across age groups.

The studies indicate that an exposure percentage of 40 to 60% is required for simultaneous bilingual children to score within the monolingual normal range, and suggest that a similar relationship holds for sequential bilinguals. For school-age children, the findings provide clear evidence that amount of exposure exerts a stronger influence on performance than does timing. Further, they show that for school-age children, both simultaneous and sequential children score significantly lower than monolingual children in important areas of language. The findings call into question the traditional separation between simultaneous and sequential bilingual children. Further, the findings underscore the low L2 performance of a large group of children, calling into question the idea that bilingualism is associated with benefits for all participants.